In mobile communication devices, such as cellular phones, a power amplifier circuit is used for amplifying power of a radio frequency (RF) signal to be transmitted to a base station. As an amplifier element of a power amplifier circuit, a bipolar transistor, such as a heterojunction bipolar transistor (HBT), is used.
When a bipolar transistor is driven with a constant base-emitter voltage, a collector current increases due to a temperature rise. An increased collector current increases power consumption, which raises the temperature of the transistor element and further increases the collector current. This is called thermal runaway (positive feedback). It is thus desirable to suppress the occurrence of thermal runaway of a bipolar transistor used in a power amplifier circuit. Japanese Unexamined Patent Application Publication No. 2006-147665 discloses the following configuration for suppressing the occurrence of thermal runaway. In order to transfer a temperature change in a bipolar transistor to a temperature control element, thermal conduction wiring using a high thermal conductive metal is provided so that the bias voltage to be supplied to the bipolar transistor can be controlled.